1. Field of the Invention
The present invention relates to a smoke alarm activated light and, more particularly, to a portable light mountable on a wall which is illuminated in response to the audible signal of a smoke alarm device.
2. Description of the Prior Art
Smoke alarm devices have come into widespread use in both residential and commercial establishments in order to alert the occupants to take emergency action in the earliest stages in the development of a fire. By signalling a loud, audible alarm upon the detection of a fire, a structure can be evacuated in an orderly manner. In the daytime, there is generally no problem in doing so because there is adequate light for one to see their way. However, if the occupants of a building are aroused from sleep by an alarm from a smoke detector, there is likely to be confusion and the possibility of an inability to see in attempting to escape from the building. Further, injuries from obstacles unseen in the darkness are a possibility as the occupants move about in attempting to find light switches to evacuate the structure.
In response to this additional need, portable lamps have been developed which are illuminated in response to the sound of a smoke alarm whereby evacuation from a burning building at night is made more safe. Such devices provide a portable light for emergency illumination which is activated in response to the sound emitted by a smoke alarm device. Such a smoke alarm activated portable light is disclosed and claimed in U.S. Pat. No. 4,258,291 issued Mar. 24, 1981, to Robert J. Scott et al.
It is much preferred that such a light be battery operated. This substantially simplifies the light, eliminates the need for various approvals, and permits it to be mounted in any convenient location. On the other hand, making the light battery operated creates the significant problem of designing a system that will provide a significant period of battery life. With current technology and the circuitry generally thought necessary for interconnecting a microphone responsive to the smoke alarm sound signal, the light, and the battery, the current drain would run the battery down within a matter of weeks if the circuit was permitted to operate at all times.
As a result, the smoke alarm activated portable lamp of the Scott et al patent includes light activated circuitry, including a photocell, for rendering the lamp unresponsive to a sound signal during illumination of the light activated circuitry to thereby conserve battery power. The photocell drives an amplifier which is operative to saturate or overdrive the first gain stage of the light activating circuitry to override any signal from the microphone whereby the lamp is rendered unresponsive to the smoke alarm signal upon illumination of the photocell by daylight or artificial light.
The obvious theory of the Scott et al patent is that when there is adequate light to permit the occupants of the building to safely exit therefrom, the portable light may be rendered unresponsive to a smoke alarm sound signal to conserve battery power. However, this has proven to be an unsatisfactory solution to the problem for a variety of reasons. First of all, there are occasions when a room does not receive daylight or artificial illumination for a significant period of time during each day so that the battery wears out very quickly. There are also instances when on a dark day or depending on where the device is located, there is a low enough light level that the circuit will turn on and draw power in a situation that would be unnecessary. Therefore, there are many instances when the circuit is unnecessarily draining the battery. Furthermore, even under normal operating circumstances, the circuit is on all night long so that the battery life has proven to be quite short.
Another problem with using a photocell activated battery saving circuit is in calibration of the photocell. Based upon the wide variety of light levels within a home, the sensitivity of the photocell becomes a significant factor, decreasing the chances of a reliable system.
In the application of John S. Skarman et al, Ser. No. 424,584, filed Sept. 27, 1982 and entitled Smoke Alarm Activated Light, which application is assigned to Gateway Scientific, Inc., the assignee of the present application, there is disclosed a portable light for emergency illumination which includes a switch interconnecting a battery and a light bulb for selectively activating the light bulb and a circuit interconnecting a microphone and the switch for selectively activating the switch in response to a smoke alarm sound signal. According to such application, the circuit which interconnects the microphone and the switch need not be on all of the time. It is satisfactory if the circuit is periodically turned on, for a short period of time, to determine whether the microphone is receiving a smoke alarm sound signal. By removing all power from the main circuit except for a very small fraction of the time, power consumption is significantly reduced. Accordingly, the light includes a strobe circuit interconnecting the source of power and the circuit means for periodically activating the circuit means so that the circuit means is operative and drawing power only for a small percentage of the time.
Significant practical difficulties have been encountered in the use of a smoke alarm activated light incorporating a strobe circuit. The circuit is found to operate differently with microphones of different sensitivities and the on and off times of the circuit vary considerably based upon component tolerances. As a result, a wide variation in circuit operation is found, making the circuit difficult to produce in quantity and resulting in a circuit whose performance is highly unpredictable. As a result, the predicted battery life is difficult to obtain. Furthermore, because of significant variations in the off time, the circuit has varying responses to alarm sounds. Because turn-on of the circuit and activation of the light bulb causes a considerable battery drain and the battery voltage is used to establish a reference level, it is possible for the circuit to go into an oscillation mode when triggered.